There are known pressure-based protective systems situated on the apparatus side which respond quickly in the event of the connection between the patient and the arterial segment of the extra-corporeal circuit becoming detached. However, it is not always certain that the known pressure-based protective systems will respond in the event of the venous needle coming out of the vascular access. Also, in the event of a blood leak in the system of venous tubing, it may happen that the resulting fall in venous pressure is not sufficiently great to ensure that the protective systems will trip.
U.S. Pat. No. 6,221,040 B1 describes an arrangement for monitoring a vascular access with which the slipping-out of both the arterial needle and the venous needle can be detected more reliably. To monitor the vascular access, the pressures in both the arterial segment of the extra-corporeal blood circuit and its venous segment are monitored by means of pressure sensors. Values characteristic of the state of the vascular access are calculated from the arterial and venous pressures in a calculating unit and these are analysed in an analysing unit to allow a faulty vascular access to be detected. To enable the values characteristic of the state of the vascular access to be calculated, the sum and the difference of the venous and arterial pressures in the extra-corporeal circuit can be determined.
As well as the above method in which the pressure in the arterial and venous segments of the extra-corporeal blood circuit is monitored, there are also known monitoring arrangements which are based on the monitoring of pulses of pressure which propagate in the extra-corporeal circuit.
DE 101 15 991 C1 (US 2002/0174721 A1) describes a method for detecting stenoses in a hose line system during an extracorporeal blood treatment, wherein an oscillating pressure signal is generated in the hose line system and the oscillating pressure signal is measured. The frequency spectrum of the oscillating pressure signal is analysed for the purpose of detecting stenoses, it being concluded that there is a stenosis when there is a change in the frequency spectrum. For this purpose, a Fourier transform of the oscillating pressure signal in the venous branch of the extracorporeal circuit is carried out. The static part is extracted from the Fourier spectrum of the venous pressure signal, wherein the attenuation of at least one harmonic oscillation of the pressure signal is ascertained and it is concluded that there is a stenosis from the change in the attenuation.